A trend in optical communication systems is to integrate functions traditionally performed by discrete optical components onto an integrated optic ("IO") device. These functions include polarizing, filtering, modulating, etc. One such IO device, having utility in the communications field, is operative to modulate an optical signal. Such an IO device is typically fabricated from a substrate of lithium niobate, LiNbO.sub.3, or lithium tantalate, LiTaO.sub.3, and has a waveguide formed on a major surface to provide parallel optic pathways. Examples of such a device, commonly referred to as an optic modulator, include a Mach-Zehnder modulator and a Balanced-Bridge interferometer.
A Mach-Zehnder includes input and output Y-junctions, parallel waveguide arms, and electro-optic modulators. An input optic signal such as a laser beam is split at the input Y-junction in the waveguide into two equal components. Each component travels in a corresponding arm of the waveguide before being recombined at the output Y-junction. To modulate the optic signals in the Mach-Zehnder, one or more electro-optic modulators comprising electrodes are formed on the waveguide surface in the vicinity of the arms. A time varying voltage, e.g., a radio frequency ("RF") signal, applied to the electrodes produces an electric field in the IO device substrate.
The basic operating principle of all optic modulators is the same. In accordance with the well-known electro-optic effect, an electrical field produced by an electrical input ("modulating") signal effectively changes the relative indices of refraction and thus changes the optic path lengths of the waveguide arms. Modulation of an optic input signal occurs since the relative phase of the optic signals in the arms varies according to the instantaneous amplitude of the time varying modulating signal driving the electrodes. The varying phase results in a varying amplitude of the intensity of the optic signal at the output of the modulator.
Since a modulator operates on light interference principles, its transfer function is a sine curve, i.e., sin(X). Consequently, a modulator generates undesirable harmonics when driven by a modulating signal such as an RF signal. When the modulator is driven symmetrically about the optical half intensity point of the sine curve, i.e., the approximately linear region of the curve, fundamental and odd harmonics predominate in the modulator's output. The amplitudes of the higher harmonics increase as the modulating voltage is increased. Thus, the modulator's output deviates noticeably from that which would be obtained with a perfectly linear device. See Donaldson, A. et al., "Linearity Considerations in High Performance Amplitude Modulators", IEEE Colloquium on `Analogue Optical Communications`, Digest No. 156, pp. 4/1-5, December 1989.
Such harmonic intermodulation distortion due to odd harmonics is a problem in a multichannel optical communication system, e.g., cable television ("CATV"), where an RF signal is used to modulate an optical signal. A CATV system may have 80 channels multiplexed for transmission in a frequency range of 50-600 MHz. The large number of closely spaced carriers places strict requirements on the linearity of system elements in order to reduce undesired harmonic intermodulation distortion.
Typical amplitude modulation ("AM") transmission requirements for second and third order harmonics are -65 dBc relative to the fundamental. The inherent sine response of an optic modulator does not provide the requisite linearity. One means of lessening the effect of harmonic intermodulation distortion is to decrease the carrier modulation depth, known as the optical modulation index ("OMI"). However, this is an inefficient use of the optical power. This reduction in OMI has been taught by Donaldson. Attempts at linearizing the modulator's response have been made so as to reduce the amplitude of the higher order distortions. Such restriction reduces the transmitted signal strength. Thus, to improve the signal to noise ratio it is necessary to increase the transmitted optic power, which requires a more expensive optic source.
It is known in the art to use directional couplers, alone or in combination with Mach-Zehnders, to reduce harmonic intermodulation distortion. Such schemes employ exponential or other trigonometric terms to reduce the second and third order terms. See Lin, Z. -Q. et al., "Waveguide Modulators with Extended Linear Dynamic Range--A Theoretical Prediction", IEEE Photonics Technology Letters, Vol. 2, No. 12, pp. 884-886, December 1990; Liu, P. -L. et al., "In Search of a Linear Electrooptic Amplitude Modulator", IEEE Photonics Technology Letters, Vol. 3, No. 2, pp. 144-146, February 1991. However, creating a parallel optic structure requires complex phase and amplitude adjustment schemes. Further, coherent addition of correction terms requires the difficult task of maintaining the optic phase alignment of one or more parallel branches.
Optic linearization schemes are known in which two parallel Mach-Zehnders achieve incoherent combination of light intensities. See, e.g. Lin, Z. -Q. et al., "Reduction of Intermodulation Distortion of Interferometric Optical Modulators Through Incoherent Mixing of Optical Waves", Electronics Letters, 1990, Vol. 26, No. 23, pp. 1980-1982. However, such a scheme may be limited to a narrow frequency range and a small optic signal transmission distance due to wavelength dispersion of the transmitted optic signal.
Commonly-owned, co-pending U.S. patent application Ser. No. 07/803,818, filed Dec. 9, 1991, now U.S. Pat. No. 5,168,534 entitled CASCADE OPTIC MODULATOR ARRANGEMENT discloses a cascade linearization circuit to reduce harmonic intermodulation distortion. In the cascade circuit, two optic modulators, such as Mach-Zehnders, are connected in series. A signal fed to the optic input of the first Mach-Zehnder is modulated by an electro-optic modulator. The resulting modulated optic signal is fed to the optic input of a second Mach-Zehnder and is modulated by the electro-optic modulator of the second Mach-Zehnder. The phase offset of both modulators, and the contrast of one modulator, are adjusted to minimize both second and third order harmonics. However, the cascade arrangement requires phase and gain accuracy to be tightly constrained. Further, the arrangement is costly because two modulators are required.
It is also known in the art to use predistortion to compensate for the optic modulator's non-linear transfer function. Predistortion refers to a technique of distorting a modulating signal equally in phase but opposite in amplitude to the transfer function of the optic modulator before feeding the modulating signal to the optic modulator. Thus, the predistortion effectively cancels the distortion inherent to the optic modulator. Prior art predistortion circuits employ a simple diode network to directly approximate the transfer function arcsin(X). Arcsin(X) is used since the transfer function of the optic modulator is sin(X), i.e., sin[arcsin(X)]=X. However, such prior art systems have severe limitations. Such systems drift and operate over relatively narrow bandwidth. Further, high impedance and current biasing sources are needed.